1. Field of the Invention
The present invention relates to a two-stroke internal combustion engine in which an intake valve and an exhaust valve are arranged in a cylinder head.
2. Description of the Related Art
Japanese Examined Patent Publication (Kokoku) No. 60-5770 discloses an open-chamber type two-stroke internal combustion engine with an intake valve and an exhaust valve. In this two-stroke engine, the intake valve and the exhaust valve open simultaneously when the piston moves to the bottom dead center, to carry out a scavenging action; namely, fresh air entering the combustion chamber from the intake valve first flows downward along the vertical wall portion of the combustion chamber, and is then deflected by the top of the piston to move upward along the opposite vertical wall portion, to describe a vertical U-shaped course, and accordingly, the location of an interface between the fresh air and exhaust gas is changed, initially near the intake valve, and is moved toward a lower central part of the combustion chamber and near the exhaust valve, and thus exhaust gas is entirely displaced by fresh air in the combustion chamber.
This two-stroke engine may perform well under a high load condition, but suffers from a poor combustion at idling and under a partial load condition. In the two-stroke engine, a small amount of fresh air is supplied at idling and under a partial load so that a large amount of the burnt gas remains in the combustion chamber and is not exhausted, and a small amount of fresh air is spread widely in the unexhausted burned gas and is not collected near the spark plug. This applies in particular to a two-stroke engine having the above stated vertical U-shaped flow, in which a substantial portion of the fresh air may move toward the lower portion of the combustion chamber and stay there. Therefore, the initial burning by the spark plug provided on the cylinder head is hindered and/or the formation of a combustion flame core is prevented. Accordingly, due to the decrease in the speed of transmission of the flame, a misfire or insufficient combustion may occur.
It is conventionally known in the art to generate a swirl about an axis of the combustion chamber. For example, U.S. Pat. No. 4,543,928 discloses a two-stroke engine in which two intake valves are arranged at opposite sides to cause a swirl of intake air about the cylinder axis. An exhaust valve in this engine is arranged in a subcombustion chamber provided centrally at the top of the combustion chamber. In this engine, combustion is initiated at the subcombustion chamber and then spread to the main combustion chamber in which a swirl is generated. This is different from an idea in which a swirl is generated by a back flow of exhaust gas and/or stratification is established between the supplied fresh air and the remaining burned gas in the combustion chamber.
As stated above, conventional two stroke internal combustion engines generally suffer from a problem of a poor combustion at idling and under a low load, since the exhaust action in two stroke internal combustion engines is carried out together with an intake action, to effect a scavenging action, so that a considerable amount of burned gas remains in the combustion chamber and the ratio of supplied fresh air to the remaining burned gas becomes low, and thus the combustion becomes unstable.
The applicant (assignee) for the present invention previously proposed a two-stroke internal combustion engine in which combustion is carried out in a stratified gas between supplied fresh air and remaining burned gas to allow fresh air to collect near the cylinder head at idling and under partial load, by generating a swirl by a portion of the exhaust gas about an axis of the combustion chamber that flows back from the exhaust port into the combustion chamber after being once exhausted through the exhaust valve.